LONDON -- London police say a second man has been arrested in connection with the Sept. 15 subway attack in the city's subway.

Police said Sunday that a 21-year-old man was arrested late Saturday night in Hounslow in west London and is being held under the Terrorism Act. He is being questioned at a south London police station but has not been charged or identified.

Two men are now in custody for possible roles in the bombing attack on a rush-hour subway train Friday morning that injured 29 people in London. An 18-year-old man was arrested Saturday in the departure area of the port of Dover, where ferries leave for France.

The two arrests indicate police and security services believe the attack at the Parsons Green station was part of a coordinated plot, not the act of a single person.

"We are still pursing numerous lines of enquiry and at a great pace," counter-terrorism coordinator Neil Basu of the London police said late Saturday.

Britain's terror threat level remains at "critical" -- the highest level -- meaning that authorities believe another attack is imminent. The official threat level is not likely to be lowered until police believe all of the plotters have been taken into custody.

Police on Saturday launched a massive armed search in the southwestern London suburb of Sunbury. Neighbors were evacuated in a rush from the area and kept away for nearly 10 hours before they were allowed to return to their homes.

The Islamic State says the attack Friday was carried out by one of its affiliated units. The improvised explosive device placed on the subway train only partially detonated, limiting the number of injuries.

Home Secretary Amber Rudd said the casualties would have been far higher if the bomb had fully detonated. Frustrated by the string of terrorist attacks in recent months, she said officials will have to work harder to make bomb components more difficult to obtain.

Britain has endured four other attacks this year that have killed 36 people. The other attacks in London -- near Parliament, on London Bridge and near a mosque in Finsbury Park, north London -- used vehicles and knives to kill and wound.